In a heat exchanger disclosed in U.S. Pat. No. 5,242,015, an extruded tube has a plurality of passages. The passages are arranged in a row parallel to a major axis of the tube cross-section. The extruded tube is layered or wound. In this kind of heat exchanger, heat transmission efficiency is likely to be lessened due to voids between surfaces of the layered tube.
Also in U.S. Pat. No. 5,242,015, an extruded tube in which three rows of passages are formed is proposed. In this kind of tube, in a case that the passages are defined into substantially triangular cross-sectional shapes, it is difficult to form walls between the passages in adjacent rows.
For example, as shown in FIG. 10, when a tube in which passages are defined in rows is extruded, an extrusion material flowed between dies in a minor direction of the tube cross-section has to change its flow direction (arrows T) into a major direction of the tube cross-section to reach middle portions S. Therefore, it is difficult to fill between the dies adjacent to the minor direction with the extrusion material.